In certain electrostatographic imaging and recording processes, for instance, in electrophotographic copying processes, an electrostatic latent image formed on a photoconductive surface is developed with a developer which is a mixture of carrier particles, e.g., magnetic particles, and a thermoplastic toner powder which is thereafter fused to a receiver such as a sheet of paper. The fusing step commonly consists of passing the substrate, such as a sheet of paper on which toner powder is distributed in an imagewise pattern, through the nip of a pair of rolls, at least one of which is heated. A persistent problem in this operation is that when the toner is heated during contact with the heated roll it may adhere not only to the paper but also to the fusing member. Any toner remaining adhered to the member can cause a false offset image to appear on the next sheet and can also degrade the fusing member. Another potential problem is thermal degradation of the member surface which results in an uneven surface and defective patterns in thermally fixed images.
Toner fusing rolls have a cylindrical core which may contain a heat source in its interior, and a resilient covering layer formed directly or indirectly on the surface of the core. Roll coverings are commonly fluorocarbon polymers or silicone polymers, such as poly(dimethylsiloxane) polymers, of low surface energy which minimizes adherence of toner to the roll. Frequently release oils composed of, for example, poly(dimethylsiloxanes), are also applied to the roll surface to prevent adherence of toner to the roll. Such release oils may interact with the roll surface upon repeated use and in time cause swelling, softening and degradation of the roll. Silicone rubber covering layers which are insufficiently resistant to release oils and cleaning solvents are also susceptible to delamination of the roll cover after repeated heating and cooling cycles.
Fusing rollers currently used in fusing toners can be of multilayered construction. A two-layer fusing roller frequently comprises a cylindrical core covered with a silicone elastomer layer and coated thereon a fluoroelastomer. Fluoroelastomer surface layers, which have a propensity to interact with toners and cause premature offsets, are used with releasing oils and are resistant to penetration by the oils. A multilayered roll frequently comprises a cylindrical core, covered with a silicone elastomer coated with a fluoroelastomer intermediate layer which serves as an oil-barrier layer to preserve the underlying silicone elastomer, and coated on the fluoroelastomer layer a thin surface layer of a silicone elastomer. The surface layer thus has the advantageous releasing properties of the silicone elastomers and offset is minimized. However, due to marginal adhesion of fluoroelastomer layers to silicone elastomers, the inner silicone elastomer surface must be first exposed to a high voltage discharge, known as corona discharge treatment (CDT), before a thin coating of fluoroelastomer is applied. Similarly the fluoroelastomer layer must be exposed to CDT before a silicone surface layer is applied. Such treatment is inefficient and unacceptable adhesion of the current fluoroelastomer overcoats or interlayers is common and adds to the overall cost of fabrication of such rolls.
U.S. Pat. No. 4,264,181 discloses fusing members coated with a metal-filled elastomer surface obtained by nucleophilic-addition curing of a mixture of a metal filler and a vinylidene fluoride-hexafluoropropylene copolymer. Mixtures of the fluoroelastomers with silicone rubbers are also contemplated (Column 8, Lines 26-29) but no specific examples of suitable silicones are taught. The surface coatings are used in conjunction with functionally substituted polymeric release agents capable of interacting with the metal component.
U.S. Pat. No. 4,853,737 discloses a roll useful in electrostatography having an outer layer comprising a cured fluoroelastomer containing pendant polydiorganosiloxane segments that are covalently bound to the backbone of the fluoroelastomer.
There is still a need for coating compositions to provide a fusing roll with an intermediate oil-barrier layer which adheres strongly to a silicone elastomer layer, preferably without requiring CDT processing of the underlying surface.